If you want to override the JSONObject of the jQuery UI widget, you need to use the CustomDatepicker mixin, provided by the Tapestry5-jQuery project. This mixin has just one JSONObject-type parameter params, which correspond to the parameters of the jQuery UI widget. You will find more information on the Tapestry5-jQuery website : http://tapestry5-jquery.com/docs/mixins/customdatepicker

You should not use the dateFormat parameter of the jQuery UI widget. The Java and jQuery formats are not the same. We have created an utility method, that will convert a Java format into a jQuery one. If you need to specify a specific format, you should use the format parameter of the datefield component.